如何在 MYSQL LEFT JOIN 中选择最新的 DATE

Question

i have 2 MYSQL tables that i LEFT JOIN based on the machine id column ('mid')

machines' table

+-----+-------------+-------------+-----+---------------------+
| mid | machineName |  ipAddress  | nid |      modified       |
+-----+-------------+-------------+-----+---------------------+
|   1 | machine1    | 192.168.1.1 |  16 | 2018-01-28 19:48:53 |
|   2 | machine2    | 192.168.2.1 |  16 | 2018-01-29 16:28:11 |
+-----+-------------+-------------+-----+---------------------+

'user_reverts' table

+----+-----+-----+---------------------+
| id | uid | mid |      modified       |
+----+-----+-----+---------------------+
| 11 | 189 |  1  | 2018-01-30 10:01:10 |
| 13 | 189 |  2  | 2018-01-30 12:11:50 |
| 14 | 190 |  1  | 2018-01-26 17:09:17 |
| 15 | 190 |  2  | 2018-01-25 15:51:31 |
| 16 | 189 |  2  | 2018-01-30 19:48:58 |
+----+-----+-----+---------------------+

the query counts the number of reverts the user has made on each machine since the beginning of each day and groups it by machine.

SELECT machines.mid as mid, machines.machineName as machineName,
        machines.ipAddress as ipAddress, coalesce(reverts.count, 0) as count, 
        coalesce(reverts.modified,'never') as reverted FROM
            (SELECT mid, machineName, INET6_NTOA(ipAddress) as ipAddress FROM machines) machines
            LEFT JOIN
            (SELECT machines.mid,user_reverts.modified,count(*) as count FROM machines
            INNER JOIN user_reverts ON user_reverts.mid=machines.mid AND user_reverts.uid = 189
            WHERE DATE(user_reverts.modified) = CURDATE() group by user_reverts.mid) reverts 
            ON reverts.mid = machines.mid

it works well except i want the 'reverted' column to show the latest time and date, so the expected result would be

+-----+-------------+-------------+-------+---------------------+
| mid | machineName |  ipAddress  | count |      reverted       |
+-----+-------------+-------------+-------+---------------------+
|   1 | machine1    | 192.168.1.1 |     1 | 2018-01-24 10:01:10 |
|   2 | machine2    | 192.168.2.1 |     2 | 2018-01-30 19:48:58 | (latest)
+-----+-------------+-------------+-------+---------------------+

i tried to replace

coalesce(reverts.count, 0)

with

MAX(reverts.count, 0)

but i get an error message. how do i do this right ? Thanks

Answer 1

You can add another column and simplify your query allalong:

        SELECT machines.mid as mid
         , machines.machineName as machineName
         , INET6_NTOA(ipAddress) as ipAddress
         , coalesce(count(*), 0) as count 
         , coalesce(MAX(user_reverts.modified),'never') as reverted  -- latest revert 
      FROM machines
 LEFT JOIN user_reverts
        ON ( user_reverts.mid=machines.mid AND user_reverts.uid = 189 )
     WHERE DATE(user_reverts.modified) = CURDATE()
  GROUP BY machines.mid
         ;

Have a look at this SQLFiddle (The exact table definitions may differ from yours, of course).